ShaanxiYanchang Petroleum (China)

Abstract Due to the tight sandstone widespread development of nanoscale pore-throat systems, the microscopic pore-throat characteristics of tight reservoirs are the focus of research. Taking the main tight oil production province of the Ordos Basin in China as a case study, nuclear magnetic resonance (NMR), pressure-controlled mercury injection (PMI), scanning electron microscopy, and micro-computed tomography (μCT) are used to analyze the characteristics of four types of tight sandstones with different pore throats. Furthermore, the effects of pore throats on movable fluid, connectivity, and reservoir physical properties are analyzed. The results show that the pore throats of the four types of tight sandstones are obviously different, but they are all dominated by nanoscale pore-throat systems. From types I to IV, the pore types of the tight sandstone change from residual intergranular pores to the coexistence of feldspar dissolved pores and residual intergranular pores and then to the coexistence of residual intergranular pores and intergranular micropores. The T 2 NMR spectrum changes from a double peak to a single peak, the pore-throat connectivity revealed by μCT decreases from 83.8% for type I to 13.1% for type IV, the pore-throat volume ratio revealed by PMI decreases from 2.09 to 0.43, and the NMR movable fluid saturation decreases from 38.91 to 12.39%. Tight sandstone with larger pores and a uniform distribution has high movable fluid saturation and good pore-throat connectivity. Although the tight sandstone with dissolved pores and intergranular pores may have a medium porosity, the pore–throat connectivity deteriorates compared with high porosity sandstone. Large pore throats, which account for less than 15% of all pore throats, contribute more than 90% of the permeability, while pore throats less than 0.8 μm in diameter are major contributors to the reservoir space, and both the porosity and permeability decrease as the pore-throat diameter decreases.

Metasurface energy harvesters (MEHs) lessen the dependence of wireless communication devices on batteries or other external power sources by capturing untapped electromagnetic energy in the surroundings. In this paper, we propose a multi-frequency and multi-mode microwave metasurface for efficient radio frequency (RF) and microwave energy harvesting. The MEH is comprised of a sub-wavelength resonant ring array, which can harvest RF energy in both Wi-Fi and 5 GHz bands. A feeding network is designed to integrate the TE and TM wave energy collected by the MEH into two separate networks that each gather AC energy and deliver them to the resistive loads. In terms of the simulation’s results, the efficiency of energy harvesting at frequencies of 2.4 GHz, 3.1 GHz, as well as 3.6 GHz is 91.3%, 88.9%, and 73.9%, specifically. We manufactured a 6 × 6 array sample and conducted experiments utilizing a microwave anechoic chamber. The simulation results and results from experiments were approximately identical. The proposed design has potential applications in various fields, such as efficient wireless energy harvesting systems, self-powered devices, which has a significant potential on the environment and the energy sector by reducing carbon emissions and reducing reliance on non-renewable energy sources.

Abstract Mud loss, the most serious formation damage in oil and gas well drilling engineering, is also an unresolved technical issue. To prevent mud loss, it is necessary to accurately understand and identify three key factors of mud loss, including the location of the loss, the time of occurrence, and the severity of the loss. The diagnosis of mud loss is a prerequisite for the proper formulation of mud loss control techniques. It emphasizes the integration of pre-drilling, drilling and post-analysis information to describe, characterize and predict potential loss zones. A mathematical model for loss probability evaluation was developed based on the theory of engineering fuzzy mathematics. It combines logging anomaly features and engineering data to predict the location of losses from drilling mud, and develops a loss formation identification method. Hydraulic fracture deformation is predicted by stress-sensitive experiments and numerical simulations, and the severity of the loss channels is quantitatively evaluated. Loss pressure models have been developed based on mud loss mechanisms, and loss pressures for hydraulic fracture creation, connection and extension have been applied to identify loss mechanisms and types. Mud losses can be identified by unusual engineering characteristics, including sudden changes in drilling times, cuttings, and mud logging. Real-time logging parameters can be used to monitor the loss process and predict loss trends. A framework for loss diagnosis techniques has been developed, which can help in successful mud loss control.

Ordos Basin is a multicycle superimposed cratonic basin in northern China with a simple internal structure and a gentle stratigraphic slope. It is characterized by extensive development, numerous oil and gas-bearing layers, and complete source-reservoir-caprock assemblages. However, the research on continental shale reservoirs and gas-bearing characteristics in the southern part of the basin is not in-depth enough, which restricts the further exploration and development of shale gas in this area. In this paper, the development characteristics of pores and fractures in shale reservoirs and the gas-bearing characteristics of Yanchang Formation shale in the study area are summarized by the comprehensive use of field outcrop, core, and logging, combined with common thin section observation, scanning electron microscope observation, X-ray diffraction analysis, and other methods and techniques. Research indicates that the cement content of the sandstone in the Yanchang Formation reservoir in the south of Ordos is generally 5%-25%. The reservoir also has a large number of primary pores, secondary pores, and microfractures. The gas content of the Yanchang Formation shale represented by Chang 7 is related to both organic matter and pore structure.

Problems such as well loss and collapses in deep shale gas drilling are most often due to the development of cracks in the shale formation, resulting in significant leaks of drilling fluid, the sticking and burrowing of drilling tools, and other engineering accidents. In addition, the horizontal sections of wells are very long and issues of friction, rock transport, and formation contamination loom large. As a result, the performance of drilling fluids directly affects drilling efficiency, engineering accident rates, and reservoir protection effects. We first analyze the mechanisms of each emulsifier in an oil-based drilling fluid formulation and the filtration reduction mechanisms, taking into account the collapse-prone and abnormally high-pressure characteristics of shale formations. We undertake an experimental evaluation and optimization of polymeric surfactants, such as primary and secondary emulsions for high-performance oil-based drilling fluids. The design of rigid and deformable nano-micron plugging materials with a reasonable particle size range was achieved, and we obtained a low Oil—Water ratio and high-density oil-based drilling fluid system, with temperature resistance of 200 °C, an Oil—Water ratio as low as 70:30, compressive fracturing fluid pollution of 10%, and a maximum density of 2.6 g/cm3. The reuse rate reached 100%. The developed oil-based drilling fluid system with strong plugging, a high density, and a low Oil—Water ratio suitable for deep shale gas can effectively seal the well wall, reduce liquid invasion, prevent the wall from collapsing, reduce mud leakage, reduce the consumption of oil-based drilling fluid, improve the utilization rate of old mud, and reduce drilling costs.

In the process of volumetric fracturing of shale oil, physical property changes caused by stress changes will affect the recovery, so it is urgent to conduct an analysis and evaluation of physical property changes caused by stress changes. In this research, a physical simulation experiment with constant confining pressure and variable pore pressure combined with an online nuclear magnetic resonance (NMR) technique was constructed. The results show that the increased range of the macropore (IRM) and the total pore (IRT) of matrix samples is 11.70~13.24% and 10.54~14.27% with the increase of pore pressure, respectively. That of fractured samples are 7.94~8.48% and 9.21~10.51%. It indicates that increasing pore pressure is more effective for matrix reservoirs. The decreased range of the macropore (DRM) and the total pore (DRT) of matrix samples is 10.29~13.89% and 10.38~21.24% with the unloading of pore pressure, respectively. That of fractured samples are 8.49~9.66% and 10.91~11.10%. It indicates that unloading pressure has less influence on fractured reservoirs. The increased range of permeability of matrix rock samples is 99~120% with the increase of pore pressure. Greatly supplementing formation energy is an effective means to improve the permeability of matrix reservoirs. Compared with matrix reservoirs, unloading pore pressure is more damaging to fractured reservoirs.

Evaluation of tight oil reservoir properties is of great significance to the exploration of oil and gas in tight reservoirs. The Chang 1 Member of the Yanchang Formation in the Wanhua Area, Ordos Basin is a new exploration stratum for tight sandstone oil. The lack of understanding of reservoir characteristics and crude oil enrichment rules has seriously restricted the efficient development of oil and gas resources in this stratum. In this study, the reservoir characteristics of the Chang 1 Member in the Wanhua area and the effects of superimposed sand bodies, structures and paleogeomorphology on accumulation of hydrocarbons were systematically studied. The Chang 1 sandstone is a typical ultra-low porosity-ultra-low permeability reservoir, and it has experienced destructive diagenesis of mechanical compaction, pressure solution and cementation, and constructive diagenesis of dissolution. Strong pressure solution caused the secondary enlargement of quartz and feldspar and the formation of patchy dense mosaic structures. The target layer has experienced argillaceous, siliceous and carbonate cementations. Moreover, the sandstone reservoir in the Chang 1 Member also experienced strong dissolution, and it is the main factor for the formation of secondary pores and the improvement of reservoir physical properties. The study also found that the main types of pores in the Chang 1 Member are intergranular dissolved pores and remaining intergranular pores. Superimposed sand bodies, nose-shaped uplifts, dominant facies and eroded paleo-highlands have significant effects on the hydrocarbon accumulation. Based on this study, it was found that the migration and accumulation mode of hydrocarbons in the Chang 1 reservoir belongs to the ladder-like climbing migration + structural ridge accumulation type. In addition, sand body thickness is an important controlling factor for the hydrocarbon accumulation. At present, the discovered crude oil in the Chang 1 Member is always distributed in the areas with thick sand bodies (>20 m), and most of the sand bodies have a thickness in the range of 25–40 m, and the effective thickness is in the range of 2–6 m. In addition, the eroded highlands are the highest topographic units, they are favorable areas for the large-scale accumulation of oil and gas.